# Life Cycle of a Star.

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Life Cycle of a Star

We now know that stars move through a complex life cycle – they are created, live extremely long lives and then expire. By studying different stars in various stages of development, astronomers have now established a detailed process for their life cycle.

Protostar and the Nebula
A nebula is a cloud of dust and gas, composed primarily of hydrogen (97%) and helium (3%). Gravity causes this dust and gas to “clump” together.

Protostar and Nebula A protostar is a star in its embryonic stage, and although it glows due to the release of gravitational energy, it is not yet hot enough to produce nuclear reactions within its center.

As the protostar continues to collapse due to gravity, it will attract more atoms and continually increase in mass and density. The increased density and gravity will cause the core temperature to eventually rise.

The falling atoms of gas speed up as they reach the center
The falling atoms of gas speed up as they reach the center. As they speed up they collide with each other and heat up.

Why is hydrogen so reactive?
This is why…… Hydrogen has only 1 electron in its outer most shell. In order for it to be “stable” it needs another electron. For example: water = H2O Watch the board…..so you can understand why elements react to other elements.

In order to achieve life as a star, the protostar will need to achieve and maintain equilibrium.
What is equilibrium? It is a balance, in this case a balance between gravity pulling atoms toward the center and gas pressure pushing heat and light away from the center.

Gas pressure depends upon two things to maintain it: a very hot temperature (keep those atoms colliding!) and density (lots of atoms in a small space). There are two options for a protostar at this point:

Option 1: If a critical temperature in the core of a protostar is not reached, it ends up a brown dwarf. This mass never makes “star status.” Nebula = Protostar = Brown Dwarf Option 2: If a critical temperature in the core of a protostar is reached, then nuclear fusion begins. We identify the birth of a star as the moment that it begins fusing hydrogen in the core into helium.

Star is born! A star is a luminous globe of gas producing its own heat and light by nuclear reactions (nuclear fusion). Hydrogen fuses with helium in the core.

Medium-Sized Stars The main factor that shapes the evolution of a star is how much mass it began with. In other words…how big the core is! A star’s life can take different paths. A star will spend most of its life as a main sequence star.

Once the star’s core has been changed from hydrogen to helium, the core begins to shrink.
As it shrinks, the core starts to heat up again causing the outer hydrogen shell to expand and cool. Red Giant

As the Red Giant continues to use up its fuel, the pressure released does not equal the pressure of the core. Gravity wins and the star collapses inward. The star now becomes a tiny White Dwarf.

White Dwarf The matter squeezed into a white dwarf is extremely dense. Still burns, and gives off light. Once “fuel” is used up. The star dies and turns into a Black Dwarf or (dead star). Nebula = Protostar = Main sequence = Red Giant = White Dwarf = Black Dwarf

Massive Stars More mass than medium stars and continue the same life-cycle until they become Red Giants or Supergiants. Once nuclear fusion stops within the massive star, the energy is released in the form of a Supernova.

Supernova The dust and gases from the supernova forms a new nebula which may form new stars. The core of the star will explode (depending on the size) and form into a Neutron Star.

Black Holes Stars that have the mass of 10 x or more than our sun may end up as a black hole. The core is so massive that it is swallowed up by its own gravity. Nebula = Protostar = Main sequence = Red Super Giant = Supernova a) neutron star b) black hole

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